1. Technical Field
The present invention relates to an image processing system and an image forming system for applying a screen process to an input image, and a recording medium for storing a program that causes a computer to execute such process.
2. Related Art
In the color image forming system, the color reproduction in full color can be achieved by using a plurality of coloring materials. The color image is formed by superposing the corresponding color image every coloring material. In superposing respective color images, slight displacement is caused due to the precision. The displacement in the superposition causes the generation of the moire, etc. on the image.
In order to prevent the generation of the moire, etc. due to the displacement in the superposition of respective color images, normally the rotation screen in which the screen angle to the image in the screen process is changed every coloring material color is employed. The screen angle in the rotation screen is designed not to generate the moire. At that time, there are the dot shape, the line shape, and the distributed shape as the screen shape. The dot shape can design the screen angle to have a margin of 90 degree, and the line shape can design the screen angle to have a margin of 180 degree. Normally, the line shape having the wide margin in design is employed.
In contrast, in the image of each coloring material color, normally the shade is expressed by the halftone-dot area system of the screen. This halftone-dot area system is the method that produces the shade in the overall image by changing the drawing area in the infinitesimal area of a predetermined size in response to the density tone to be drawn. For example, only one or several points in the infinitesimal area are drawn in the light portion, and thus the light tone can be expressed as the overall image since such points are scattered.
However, in the case of light color, a quite small part of the infinitesimal area can be drawn and thus areas that are drawn into predetermined patterns and areas that are not drawn into the predetermined patterns alternatively appear. Therefore, when a light thin line is to be drawn, the interference between the thin line and the drawing pattern occurs. In particular, in the light thin line having the angle that is close to the screen angle, there is the problem that the thin line is not drawn in many infinitesimal areas that successively appear in the extending direction of the thin line and that the light thin line disappears as the overall image. The disappearance of such line is ready to occur as the line is thinner, or the color is lighter, or the angle of the line is closer to the screen angle.
In the Unexamined Japanese Patent Application Publication No.Hei9-191403, for example, it is set forth that the line drawing is detected from the input image signal, and then the screen of the high resolution and the tone correction γ transformation are employed in the line drawing area. However, in this literature, much importance to the tone characteristic is attached in the portion except the line drawing portion, but much importance to the resolution is attached in the line drawing. Therefore, merely the screen shape, the screen line number, etc. are switched, but the disappearance of the thin line, etc. cannot be prevented. Like this literature, in order to prevent the disappearance of the light thin line by employing the screen of the high resolution, it is impossible to avoid such disappearance by 100% unless the resolution is increased up to the basic resolution. In such case, the color reproduction of the thin line is extremely lowered.
Also, in the Unexamined Japanese Patent Application Publication No.Hei9-282471, for example, it is set forth that features of respective pixels are acquired from the information described by PDL in the controller to execute the image processing every feature. In particular, the TEXT signal flag is provided to the object that needs the resolution for characters, line drawings, profiles, etc. to switch the image processing. However, in the screen process, merely the resolution is switched such that the 400-line screen process is applied to the image signal to which the TEXT signal flag is added and the 200-line screen process is applied to other signals. Thus, it is impossible to prevent the above-mentioned disappearance of the thin line only by switching the resolution.
In addition, in the Unexamined Japanese Patent Application Publication No.Hei9-294208, for example, it is set forth that the edges are detected from the image signal being input into the image processing system, and then a plurality of screen patterns having the same mask size are switched in response to the states of the edges. In particular, the tone reproduction can be stabilized by using the dot shapes in the portions except the edge portions, and also the edges can be made smooth by using the different screen shapes in the edge portions. As a result, the unevenness can be made smooth in the edge portions of the characters and the lines. However, in this case, such approach is not effective to the case where the thin line having the angle close to the screen angle disappears, and thus it is impossible to prevent the disappearance of the thin line.